Nd—Fe—B magnets not only have excellent magnetic properties as typified by a maximum energy product about 10 times that of ferrite magnets, but are also manufactured at relatively low cost by combining iron with B and Nd which is relatively inexpensive, abundant in resource and commercially available in a stable supply. For these reasons, Nd—Fe—B magnets are utilized in a wide variety of products like electronic equipment and also employed in motors and power generators on hybrid vehicles. The demand for Nd—Fe—B magnets is increasing.
Although Nd—Fe—B magnets have excellent magnetic properties, they are less corrosion resistant because they are based on Fe and Nd, a light rare earth. Even in an ordinary atmosphere, rust forms with the lapse of time. Often Nd—Fe—B magnet blocks are covered on their surface with a protective layer of resin or plating.
JP-A H02-004939 discloses multiple substitution of Co and Ni for part of Fe as an effective means for improving the corrosion resistance of a magnet body. This approach, however, is not practically acceptable because of the problem that the magnet suffers a substantial loss of coercive force when Ni substitutes for part of Fe.